A Young Carolina Bay in North Carolina

Thousands of oval-shaped depressions, known as Carolina Bays, pockmark the coastal plain from Virginia to Georgia.  Originally, there were about 500,000 Carolina Bays on the southeastern coastal plain, but farmers fully or partially drained most of them.  They range in size from a small fraction of an acre to tens of thousands of acres.  Some stay wet year round, while others may only hold water for a few weeks of the year.  Small bays devoid of egg and tadpole-eating fish are ideal breeding grounds for amphibians.  The wetland habitats, even if seasonal, provide critical environments for aquatic species, including rare flora, such as pitcher plants.

Aerial photograph of some Carolina Bays in South Carolina.  Most geologists believe they are formed from a combination of wind and water erosion as well as peat fires which lower the elevation of the depressions. 

A minority of scientists believe meteor or comet impacts created Carolina Bays, but convincing evidence debunks this theory.  There is a complete absence of the kinds of rock that are associated with extraterrestrial impact.  Moreover, the ages of Carolina Bays vary greatly.  Not all have been dated, but some are tens of thousands of years old, while others may be as young as 6500 years.  Instead, Carolina Bays were likely formed from a combination of wind and water erosion and peat fires.

Stilted trees growing in a dried out Carolina Bay.  A fire just burned the peat causing the elevation to decrease about 4 feet.  The tree roots appear as stilts.

All Carolina Bays are oval in shape and are oriented perpendicular to the prevailing winds of the Pleistocene.  Much of the loess, or sand constituting dunes in the southeast, may have even originated from Carolina Bays.  The two geological anomalies are closely interrelated.  During dry climate phases, peat swamps became dessicated.  Lightning strikes ignited fires that burned off all the peat, thus lowering the elevation by as much as 4-5 feet.  Wind blew the exposed soil to the northeast, explaining why a sandy lip can be found on the northeast side of a Carolina Bay.  Later, after the rains returned, the water table rose and ponded water driven by wind also eroded land along the same axis.

Most Carolina Bays formed during stadials of the Pleistocene, especially the Last Glacial Maximum (~28,000 BP-~15,000 BP), but Lake Mattamuskeet, an enormous Carolina Bay near the North Carolina coast just a few miles from Pamlico Sound, is a young one with origins dating to about 6500 BP.  Scientists knew the lake had to be younger than 80,000 years old because a high stand of the Atlantic ocean inundated this area until then.  When they took cores down to that aged level, they found a layer of marine sediment, including saltwater species of clam and snail shells.  Above this layer they found peat.  Charcoal and estuarine silt (also known as loess) was mixed in with the peat.  The former is evidence of peat and forest fires; the latter was windblown from what is now Pamlico Sound, but was then high and dry land because during the Ice Age the Atlantic Ocean receded many miles to the east, leaving exposed marine-derived soil.

The scientists used ground penetrating radar, vibracores (http://www.vibracoring.com/), and radiocarbon dating to determine that the present day site of Lake Mattamuskeet was a heavily vegetated forest from about 12,000 BP-8500 BP.  Peat swamps grew in 3 areas within this site.  Periodically, they would burn, and silt from dry land or what is now Pamlico Sound would be deposited here via wind.  About 7,000 BP the ocean level rose and filled in Pamlico Sound.  Lake Mattamuskeet began forming about 6500 years BP during dry spells when peat fires left depressions of dry earth further scooped out by wind.  These peat fires still occasionally occur as the above photo shows and they can last for over a year as they slowly smolder.  An Indian legend even supports the scientific study of how Lake Mattamuskeet formed.  Supposedly, the Indian legend claims a great fire burned here for 13 moons, creating a depression that later filled with water.  According to scientists, the 3 bays stopped forming ~5,000 years ago, and the water table rose, joining the 3 lakes into 1.

Lake Mattamuskeet in North Carolina is a Carolina Bay within a few miles of the sea shore.  It’s a shallow freshwater lake though a manmade canal was constructed in 1850 in a failed attempt to drain the water into the saltwater Pamlico Sound.  In this photo it looks as deep as a reservoir, but the depth is only 2-3 feet deep.  Reportedly, there is good fishing, crabbing, and duck hunting here.  Scientists studied the geological history of this Lake and determined it’s just ~6500 years old. 

Reference:

Rodriguez, Antonio; Matthew Waters, Milas Pehler

“Burning Peat and Reworking Loess Contributes to the Formation and Evolution of a Large Carolina Bay Basin”

Quaternary Research 77 (1) Jan. 2012

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